# Symbol#to\_proc Problem **URL:** \ **Category:** ruby-talk\ **Created:** [16 June 2006 01:40 UTC](https://rubytalk.org/t/symbol-to-proc-problem/28240 "2006-06-16T01:40:48Z")\ **Posts on this page:** 11\ **Page:** 1
**Author:** ![Elliot\_Temple](https://avatars.discourse-cdn.com/v4/letter/e/c6cbf5/32.png) [@Elliot\_Temple](https://rubytalk.org/u/Elliot_Temple)\ **Post date:** [16 June 2006 01:40 UTC](https://rubytalk.org/t/symbol-to-proc-problem/28240/1 "2006-06-16T01:40:48Z")
[http://blog.hasmanythrough.com/articles/read/8](http://blog.hasmanythrough.com/articles/read/8) this link suggests a way to hack Symbol to allow [1, 2, 3].map(&:to\_s) # =\> ["1", "2", "3"] my question is how to change the hack to work correctly for other methods. details of the problem are below: it doesn't work for reversing arrays: curi-g5:~ curi$ ruby -v ruby 1.8.4 (2005-12-24) [powerpc-darwin8.5.0] irb(main):017:0\> class Symbol irb(main):018:1\> def to\_proc irb(main):019:2\> Proc.new { |obj, \*args| obj.send(self, \*args) } irb(main):020:2\> end irb(main):021:1\> end =\> nil irb(main):022:0\> [[1,2,3], [4,5,6], [7,8,9]].map {|i| i.reverse} =\> [[3, 2, 1], [6, 5, 4], [9, 8, 7]] irb(main):023:0\> [[1,2,3], [4,5,6], [7,8,9]].map &:reverse NoMethodError: undefined method `reverse' for 1:Fixnum          from (irb):19:in `send'          from (irb):19:in `to\_proc'          from (irb):23          from (null):0 the problem is that \*args is capturing part of the array. here's the same thing happening without the Symbol hack involved: irb(main):024:0\> [[1,2,3], [4,5,6], [7,8,9]].map {|i, \*args| i.reverse} NoMethodError: undefined method `reverse' for 1:Fixnum          from (irb):24          from (null):0 so how should the Symbol hack be redone to work properly? just removing \*args is no good. here is an example that needs \*args: irb(main):027:0\> class Fixnum irb(main):028:1\> def foo x irb(main):029:2\> self+x irb(main):030:2\> end irb(main):031:1\> end =\> nil irb(main):032:0\> { 1=\>2, 3=\>4}.map &:foo =\> [3, 7] so to illustrate what happens without \*args: it solves one problem while creating another: irb(main):039:0\> class Symbol irb(main):040:1\> def to\_proc irb(main):041:2\> Proc.new { |obj| obj.send(self) } irb(main):042:2\> end irb(main):043:1\> end =\> nil irb(main):044:0\> [[1,2,3], [4,5,6], [7,8,9]].map &:reverse # works now! =\> [[3, 2, 1], [6, 5, 4], [9, 8, 7]] irb(main):045:0\> { 1=\>2, 3=\>4}.map &:foo # breaks now! NoMethodError: undefined method `foo' for [1, 2]:Array          from (irb):41:in `send'          from (irb):41:in `to\_proc'          from (irb):45:in `map'          from (irb):45 -- Elliot Temple > **[Curiosity Blog](https://curi.us/blog/)** > > A philosophy blog > **···** > > from :0 ---
**Author:** ![Logan\_Capaldo](https://avatars.discourse-cdn.com/v4/letter/l/7ea924/32.png) [@Logan\_Capaldo](https://rubytalk.org/u/Logan_Capaldo)\ **Post date:** [16 June 2006 04:44 UTC](https://rubytalk.org/t/symbol-to-proc-problem/28240/2 "2006-06-16T04:44:36Z")
Yeah I don't believe it's possible to have this work both ways. You could do class Symbol    def unary\_meth      lambda { |obj| obj.send(self) }    end    def nary\_meth      lambda { |obj, \*args| obj.send(self, \*args) }    end end [[1,2,3], [4,5,6]].map &:reverse.unary\_meth { 1=2, 3=\>4 }.map &:foo.nary\_meth This is one of those cases where you can't have your cake and eat it too. > **···** > > On Jun 15, 2006, at 9:40 PM, Elliot Temple wrote: > > > [http://blog.hasmanythrough.com/articles/read/8](http://blog.hasmanythrough.com/articles/read/8) > > > > this link suggests a way to hack Symbol to allow > > > > [1, 2, 3].map(&:to\_s) # =\> ["1", "2", "3"] > > > > my question is how to change the hack to work correctly for other methods. details of the problem are below: > > > > it doesn't work for reversing arrays: > > > > curi-g5:~ curi$ ruby -v > > ruby 1.8.4 (2005-12-24) [powerpc-darwin8.5.0] > > > > irb(main):017:0\> class Symbol > > irb(main):018:1\> def to\_proc > > irb(main):019:2\> Proc.new { |obj, \*args| obj.send(self, \*args) } > > irb(main):020:2\> end > > irb(main):021:1\> end > > =\> nil > > irb(main):022:0\> [[1,2,3], [4,5,6], [7,8,9]].map {|i| i.reverse} > > =\> [[3, 2, 1], [6, 5, 4], [9, 8, 7]] > > > > irb(main):023:0\> [[1,2,3], [4,5,6], [7,8,9]].map &:reverse > > NoMethodError: undefined method `reverse' for 1:Fixnum > >         from (irb):19:in `send' > >         from (irb):19:in `to\_proc' > >         from (irb):23 > >         from (null):0 > > > > the problem is that \*args is capturing part of the array. here's the same thing happening without the Symbol hack involved: > > > > irb(main):024:0\> [[1,2,3], [4,5,6], [7,8,9]].map {|i, \*args| i.reverse} > > NoMethodError: undefined method `reverse' for 1:Fixnum > >         from (irb):24 > >         from (null):0 > > > > so how should the Symbol hack be redone to work properly? > > > > just removing \*args is no good. here is an example that needs \*args: > > > > irb(main):027:0\> class Fixnum > > irb(main):028:1\> def foo x > > irb(main):029:2\> self+x > > irb(main):030:2\> end > > irb(main):031:1\> end > > =\> nil > > irb(main):032:0\> { 1=\>2, 3=\>4}.map &:foo > > =\> [3, 7] > > > > so to illustrate what happens without \*args: it solves one problem while creating another: > > > > irb(main):039:0\> class Symbol > > irb(main):040:1\> def to\_proc > > irb(main):041:2\> Proc.new { |obj| obj.send(self) } > > irb(main):042:2\> end > > irb(main):043:1\> end > > =\> nil > > irb(main):044:0\> [[1,2,3], [4,5,6], [7,8,9]].map &:reverse # works now! > > =\> [[3, 2, 1], [6, 5, 4], [9, 8, 7]] > > > > irb(main):045:0\> { 1=\>2, 3=\>4}.map &:foo # breaks now! > > NoMethodError: undefined method `foo' for [1, 2]:Array > >         from (irb):41:in `send' > >         from (irb):41:in `to\_proc' > >         from (irb):45:in `map' > >         from (irb):45 > >         from :0 > > > > -- Elliot Temple > > [Curiosity Blog – Elliot Temple](http://www.curi.us/blog/) ---
**Author:** ![Mauricio\_Fernandez](https://avatars.discourse-cdn.com/v4/letter/m/73ab20/32.png) [@Mauricio\_Fernandez](https://rubytalk.org/u/Mauricio_Fernandez)\ **Post date:** [16 June 2006 07:26 UTC](https://rubytalk.org/t/symbol-to-proc-problem/28240/3 "2006-06-16T07:26:12Z")
FYI the "official" implementation, recently added to Ruby 1.9, works that way too: $ ./ruby19 -v -e "p [[1,2,3], [4,5,6], [7,8,9]].map(&:reverse)" ruby 1.9.0 (2006-06-11) [i686-linux] -e:1: undefined method `reverse' for 1:Fixnum (NoMethodError)         from -e:1 I read that Minero Aoki was against this being added to the core[1]; maybe it was for the reason you exposed. [1] [http://mput.dip.jp/mput/?date=20060611#p01](http://mput.dip.jp/mput/?date=20060611#p01) > **···** > > On Fri, Jun 16, 2006 at 10:40:48AM +0900, Elliot Temple wrote: > > > it doesn't work for reversing arrays: > > > > curi-g5:~ curi$ ruby -v > > ruby 1.8.4 (2005-12-24) [powerpc-darwin8.5.0] > > > > irb(main):017:0\> class Symbol > > irb(main):018:1\> def to\_proc > > irb(main):019:2\> Proc.new { |obj, \*args| obj.send(self, \*args) } > > irb(main):020:2\> end > > irb(main):021:1\> end > > =\> nil > > irb(main):022:0\> [[1,2,3], [4,5,6], [7,8,9]].map {|i| i.reverse} > > =\> [[3, 2, 1], [6, 5, 4], [9, 8, 7]] > > > > irb(main):023:0\> [[1,2,3], [4,5,6], [7,8,9]].map &:reverse > > NoMethodError: undefined method `reverse' for 1:Fixnum > >         from (irb):19:in `send' > >         from (irb):19:in `to\_proc' > >         from (irb):23 > >         from (null):0 > > -- > Mauricio Fernandez - [http://eigenclass.org](http://eigenclass.org) - singular Ruby ---
**Author:** ![Yukihiro\_Matsumoto2](https://avatars.discourse-cdn.com/v4/letter/y/b9e5f3/32.png) [@Yukihiro\_Matsumoto2](https://rubytalk.org/u/Yukihiro_Matsumoto2)\ **Post date:** [16 June 2006 07:51 UTC](https://rubytalk.org/t/symbol-to-proc-problem/28240/4 "2006-06-16T07:51:28Z")
Hi, > it doesn't work for reversing arrays: > irb(main):023:0\> [[1,2,3], [4,5,6], [7,8,9]].map &:reverse > NoMethodError: undefined method `reverse' for 1:Fixnum Good point. The ActiveSupport Symbol#to\_proc consider an array as a group of a receiver and method arguments. It was a design choice. You need to ask the rationale behind this design to the rails list. As you've stated, the alternative is making everything a receiver. I think either choice is both good and bad. Personally I prefer the latter, it's more consistent. But compatibility problem would arise.               matz. > **···** > > In message "Re: Symbol#to\_proc Problem" on Fri, 16 Jun 2006 10:40:48 +0900, Elliot Temple \ writes: ---
**Author:** ![Matthew\_Harris](https://avatars.discourse-cdn.com/v4/letter/m/51bf81/32.png) [@Matthew\_Harris](https://rubytalk.org/u/Matthew_Harris)\ **Post date:** [16 June 2006 12:35 UTC](https://rubytalk.org/t/symbol-to-proc-problem/28240/5 "2006-06-16T12:35:09Z")
You could always write a String#to\_proc to accept something like [12, 34, 56].map(&'to\_s.reverse'), but I think this is ugly. Or you may as well just do [12, 34, 56].map(&:to\_s).map(&:reverse) which is equally ugly. But of course, the traditional [12, 34, 56].map { |x| x.to\_s.reverse } way exists, so why fix something that isn't broken? > **···** > > On 6/16/06, Elliot Temple \ wrote: > > > [http://blog.hasmanythrough.com/articles/read/8](http://blog.hasmanythrough.com/articles/read/8) > > > > this link suggests a way to hack Symbol to allow > > > > [1, 2, 3].map(&:to\_s) # =\> ["1", "2", "3"] > > > > my question is how to change the hack to work correctly for other > > methods. details of the problem are below: > > > > it doesn't work for reversing arrays: > > > > curi-g5:~ curi$ ruby -v > > ruby 1.8.4 (2005-12-24) [powerpc-darwin8.5.0] > > > > irb(main):017:0\> class Symbol > > irb(main):018:1\> def to\_proc > > irb(main):019:2\> Proc.new { |obj, \*args| obj.send(self, \*args) } > > irb(main):020:2\> end > > irb(main):021:1\> end > > =\> nil > > irb(main):022:0\> [[1,2,3], [4,5,6], [7,8,9]].map {|i| i.reverse} > > =\> [[3, 2, 1], [6, 5, 4], [9, 8, 7]] > > > > irb(main):023:0\> [[1,2,3], [4,5,6], [7,8,9]].map &:reverse > > NoMethodError: undefined method `reverse' for 1:Fixnum > >         from (irb):19:in `send' > >         from (irb):19:in `to\_proc' > >         from (irb):23 > >         from (null):0 > > > > the problem is that \*args is capturing part of the array. here's the > > same thing happening without the Symbol hack involved: > > > > irb(main):024:0\> [[1,2,3], [4,5,6], [7,8,9]].map {|i, \*args| i.reverse} > > NoMethodError: undefined method `reverse' for 1:Fixnum > >         from (irb):24 > >         from (null):0 > > > > so how should the Symbol hack be redone to work properly? > > > > just removing \*args is no good. here is an example that needs \*args: > > > > irb(main):027:0\> class Fixnum > > irb(main):028:1\> def foo x > > irb(main):029:2\> self+x > > irb(main):030:2\> end > > irb(main):031:1\> end > > =\> nil > > irb(main):032:0\> { 1=\>2, 3=\>4}.map &:foo > > =\> [3, 7] > > > > so to illustrate what happens without \*args: it solves one problem > > while creating another: > > > > irb(main):039:0\> class Symbol > > irb(main):040:1\> def to\_proc > > irb(main):041:2\> Proc.new { |obj| obj.send(self) } > > irb(main):042:2\> end > > irb(main):043:1\> end > > =\> nil > > irb(main):044:0\> [[1,2,3], [4,5,6], [7,8,9]].map &:reverse # works now! > > =\> [[3, 2, 1], [6, 5, 4], [9, 8, 7]] > > > > irb(main):045:0\> { 1=\>2, 3=\>4}.map &:foo # breaks now! > > NoMethodError: undefined method `foo' for [1, 2]:Array > >         from (irb):41:in `send' > >         from (irb):41:in `to\_proc' > >         from (irb):45:in `map' > >         from (irb):45 > >         from :0 > > > > -- Elliot Temple > > [Curiosity Blog – Elliot Temple](http://www.curi.us/blog/) > > -- > Matt ---
**Author:** ![Sean\_O\_Halpin](https://avatars.discourse-cdn.com/v4/letter/s/ac91a4/32.png) [@Sean\_O\_Halpin](https://rubytalk.org/u/Sean_O_Halpin)\ **Post date:** [16 June 2006 19:54 UTC](https://rubytalk.org/t/symbol-to-proc-problem/28240/6 "2006-06-16T19:54:47Z")
[snip Elliot trying to have his cake and eat it ;)] Think about what you're trying to do here. Applying map on its own to both input data structures gives you: { 1=\>2, 3=\>4}.map #=\> [[1,2], [3,4]] [[1,2,3], [4,5,6], [7,8,9]].map #=\> [[1,2,3], [4,5,6], [7,8,9]] i.e. in both cases you are effectively mapping an array of arrays. In the &:foo case, you want to apply the proc to the first element of each array, with subsequent elements as arguments. In the &:reverse case, you want to apply the proc to each array as a whole. How can the lambda in the to\_proc method know which case is which? You might be able to devise a hack to check the arity of the methods involved, but even that is effectively guess work. On top of that, using Symbol.to\_proc in this way is very inefficient compared to using blocks. IMHO this is one of those clever hacks that shouldn't make it into production code. Regards, Sean ---
**Author:** ![Elliot\_Temple](https://avatars.discourse-cdn.com/v4/letter/e/c6cbf5/32.png) [@Elliot\_Temple](https://rubytalk.org/u/Elliot_Temple)\ **Post date:** [16 June 2006 06:29 UTC](https://rubytalk.org/t/symbol-to-proc-problem/28240/7 "2006-06-16T06:29:22Z")
that is helpful, thanks is there a reason it's important for \*args to force an array to split up, instead of just being empty? -- Elliot Temple > **[Curiosity Blog – Elliot Temple](https://www.curi.us/blog/)** > > A philosophy blog > **···** > > On Jun 15, 2006, at 9:44 PM, Logan Capaldo wrote: > > > Yeah I don't believe it's possible to have this work both ways. > > > > You could do > > > > class Symbol > >   def unary\_meth > >     lambda { |obj| obj.send(self) } > >   end > > > >   def nary\_meth > >     lambda { |obj, \*args| obj.send(self, \*args) } > >   end > > end > > > > [[1,2,3], [4,5,6]].map &:reverse.unary\_meth > > > > { 1=2, 3=\>4 }.map &:foo.nary\_meth > > > > This is one of those cases where you can't have your cake and eat it too. ---
**Author:** ![Nobuhiro\_IMAI](https://avatars.discourse-cdn.com/v4/letter/n/9f8e36/32.png) [@Nobuhiro\_IMAI](https://rubytalk.org/u/Nobuhiro_IMAI)\ **Post date:** [16 June 2006 09:09 UTC](https://rubytalk.org/t/symbol-to-proc-problem/28240/8 "2006-06-16T09:09:27Z")
Hello, > \>irb(main):023:0\> [[1,2,3], [4,5,6], [7,8,9]].map &:reverse > \>NoMethodError: undefined method `reverse' for 1:Fixnum > > Good point. The ActiveSupport Symbol#to\_proc consider an array as a > group of a receiver and method arguments. It was a design choice. > You need to ask the rationale behind this design to the rails list. > > As you've stated, the alternative is making everything a receiver. > I think either choice is both good and bad. Personally I prefer the > latter, it's more consistent. But compatibility problem would arise. > >                                                         matz. For example, I submitted [http://dev.rubyonrails.org/ticket/5295](http://dev.rubyonrails.org/ticket/5295) as follow: class Symbol   def to\_proc     Proc.new{|\*args| args.shift.\_\_send\_\_(self, \*args)}   end end > > [[1,2,3], [4,5,6], [7,8,9]].map(&:reverse) =\> [[3, 2, 1], [6, 5, 4], [9, 8, 7]] > > (1..10).inject(&:+) =\> 55 > > {0=\>"zero",1=\>"one",2=\>"two",3=\>"three"}.sort\_by(&:first).map(&:last) =\> ["zero", "one", "two", "three"] However, I wonder whether below is what we want. > > {1=\>2,3=\>4}.map(&:reverse) =\> [[2, 1], [4, 3]] Thanks, > **···** > > On 6/16/06, Yukihiro Matsumoto wrote: > -- > Nobuhiro IMAI ---
**Author:** ![James\_Edward\_Gray\_II](https://avatars.discourse-cdn.com/v4/letter/j/ea5d25/32.png) [@James\_Edward\_Gray\_II](https://rubytalk.org/u/James_Edward_Gray_II)\ **Post date:** [16 June 2006 12:57 UTC](https://rubytalk.org/t/symbol-to-proc-problem/28240/9 "2006-06-16T12:57:35Z")
I like that version better, for the official Ruby method. James Edward Gray II > **···** > > On Jun 16, 2006, at 4:09 AM, Nobuhiro IMAI wrote: > > > For example, I submitted [http://dev.rubyonrails.org/ticket/5295](http://dev.rubyonrails.org/ticket/5295) as follow: > > > > class Symbol > > def to\_proc > >    Proc.new{|\*args| args.shift.\_\_send\_\_(self, \*args)} > > end > > end ---
**Author:** ![Benjohn\_Barnes](https://avatars.discourse-cdn.com/v4/letter/b/df705f/32.png) [@Benjohn\_Barnes](https://rubytalk.org/u/Benjohn_Barnes)\ **Post date:** [16 June 2006 14:03 UTC](https://rubytalk.org/t/symbol-to-proc-problem/28240/10 "2006-06-16T14:03:26Z")
I'm intregued: Why wouldn't you simply use send? The Picaxe defines them as synonymous. I've seen arangements where all methods are stripped off an object (BlankSlate) except those prefixed with a \_. Is that the reason? Cheers,   Benjohn > **···** > > > On Jun 16, 2006, at 4:09 AM, Nobuhiro IMAI wrote: > > > > > For example, I submitted [http://dev.rubyonrails.org/ticket/5295](http://dev.rubyonrails.org/ticket/5295) as > > > follow: > > > > > > class Symbol > > > def to\_proc > > >    Proc.new{|\*args| args.shift.\_\_send\_\_(self, \*args)} > > > end > > > end > > > > I like that version better, for the official Ruby method. ---
**Author:** ![Yukihiro\_Matsumoto2](https://avatars.discourse-cdn.com/v4/letter/y/b9e5f3/32.png) [@Yukihiro\_Matsumoto2](https://rubytalk.org/u/Yukihiro_Matsumoto2)\ **Post date:** [16 June 2006 14:33 UTC](https://rubytalk.org/t/symbol-to-proc-problem/28240/11 "2006-06-16T14:33:39Z")
Hi, > **···** > > In message "Re: Symbol#to\_proc Problem" on Fri, 16 Jun 2006 23:03:26 +0900, benjohn@fysh.org writes: > > > I'm intregued: Why wouldn't you simply use send? The Picaxe defines them > > as synonymous. I've seen arangements where all methods are stripped off > > an object (BlankSlate) except those prefixed with a \_. Is that the > > reason? > > The receiver may be a socket, which accidentally has send method of > different behavior. > >               matz.